Rear projection screen with reduced speckle

ABSTRACT

A rear projection screen which includes a front lenticular surface, a diffusion region behind the lenticular surface, a non-diffusion region behind the diffusion region, and a rear phase grating surface, when used with high magnification projection systems, exhibits reduced speckle when compared to other rear projection screen without such a grating.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation of application Ser. No. 08/914,586, filed Aug.18, 1997, now U.S. Pat. No. 6,147,801, which is a divisional ofapplication Ser. No. 08/418,231, filed Apr. 6, 1995, now U.S. Pat. No.5,760,955.

This invention was made with United States Government support underAward 70NANB5H1070, entitled “High Information Content DisplayTechnology”, awarded by the Department of Commerce through its NationalInstitute of Science & Technology (NIST).

BACKGROUND OF THE INVENTION

This invention relates to rear projection screens, and more particularlyrelates to such screens for use projectors in which the projection beamis nearly coherent.

Rear projection screens transmit to an audience space an image projectedonto the rear of the screen. The performance of rear projection screensis characterized by their gain (defined as the luminance of the screenin the forward going direction with respect to the luminance from anideal lambertian reflector), their viewing space, resolution, contrastand artifacts. An ideal rear projection screen would provide a clearcrisp, high contrast, bright image for a large audience space. That is,the screen would have 1) high resolution, 2) freedom from artifacts, 3)contrast enhancement, 4) high gain and 5) spreading of the light fromthe projector into a large range of vertical and horizontal angles.

In reality, a screen that exhibits all these characteristics does notexist. For instance, to increase screen gain, the screen designer mustlimit the audience space. Typically, viewers of rear projection displaysare scattered over a large range of horizontal angles (i.e. sittinganywhere in a room) but they all fall within a limited range of verticalangles (no one is near the ceiling or directly below the screen).Therefore to increase the brightness of the system, screen gain can beincreased by limiting the vertical distribution of the light from ±90degrees to typically ±8 degrees. By ‘squeezing’ the light into thislimited audience space, the gain of the screen can be made to be as highas 6.0.

High resolution requires very small structures in the screen. Highcontrast requires a mechanism to reduce the amount of ambient room lightthat is reflected from the screen. Typically, either a black dye isadded to the screen or black stripes are added to the front surface ofthe screen to reduce reflections of ambient light. With black dye, thecontrast is increased but the gain is reduced (light from the projectoris absorbed along with the ambient light). With black stripes, theresolution is limited by the pitch of the stripes.

Therefore different screen design concepts must be used to design theoptimal screen for a specific application.

The commercial market for rear projection screens can be divided intotwo main categories; screens for consumer rear projection TVs (PTV), andscreens for specialty applications.

Almost all PTVs use double lenticular, high contrast, high gain screens.FIG. 1 shows in a longitudinal cross section a typical double lenticularscreen, described for example in U.S. Pat. No. 5,066,099. The screen ismade of two pieces. The rear piece is a Fresnel lens 10 which isgenerally designed to image the exit pupil of the projector to theviewing plane. This allows a viewer sitting on axis to see the entireimage. The front piece 12 has front 14 and rear 16 vertically orientedlenticular surfaces (one with black stripes 18) with bulk diffusionbetween the two surfaces. A bulk diffuser is defined as minute colloidalparticles 19 suspended throughout the screen. The particles (typicallyless than about 40 microns in size) have a slightly different refractiveindex than that of the screen. The bulk diffuser is designed to providethe desired vertical distribution of the image into the audience space,typically ±8 degrees.

The rear lenticular surface 16 focuses the light coming from the Fresnellens into stripes at the plane of the front surface 14. The lenticules14 a of the front lenticular surface are aligned to these stripes oflight and spread the light into a wide range of horizontal angles. Inbetween these stripes, where the screen is not optically active, stripes18 of black paint are applied. The black stripes do not effect the lightcoming out of the projector but do absorb about 50% of the ambient roomlight that hits the screen's front surface.

For this type of screen, there is a direct relationship between thepitch of the lenticular surfaces and the thickness of the screen. Aminimum thickness is required to assure mechanical durability. Thislimits the minimum pitch (distance between the lenticules) to about 0.5mm, which in turn limits the ultimate resolution of the screen.

Other screens are made for niche markets. Diffusing screens(rotationally symmetric bulk or surface diffusers) which have low gain(less than 2.0) and no contrast enhancement or dyes are used in highresolution systems such as microfilm/microfiche readers.

Non blackened lenticular screens (see FIG. 2) are typically one-piece.The Fresnel lens 20 is on the back surface and a lenticular surface 22is on the front. There are diffusing particles 19 throughout the bulk.The combination of the bulk diffuser and lenticules provide a highaspect ratio viewing space and high gain. Since there is no relationshipbetween the thickness of the screen and the pitch of the lenticules,resolution is limited only by the ability to manufacture the individuallenticular elements. The Fresnel can be put on the back surface when thedistance from the exit pupil to the screen is at least 1.33 times largerthan the diameter of the screen. Otherwise, a separate piece is neededfor the Fresnel lens. Bradley et al, IEEE Trans. on ConsumerElectronics; v. 31; (3); pp. 185-193; August 1985.

FIG. 3 shows a rear projection screen of a type called a TIR screen.Such a screen is described in U.S. Pat. No. 4,730,897, assigned to thepresent assignee, the entire contents of which are incorporated hereinby reference. These screens use bulk diffusion, sometimes confined to aregion 38 adjacent the lenticules, for vertical distribution and asingle front surface 30 for the horizontal distribution. The shape ofthe lenticules 30 a incorporates steep sidewalls 32 which totallyinternally reflect (TIRs) the light to the tip region 34 of thelenticules. The area in between the lenticules can be filled with ablackened substance 36 in a manner to maintain reflectivity of the steepsidewall surfaces, to thereby provide high contrast. This screen canhave the Fresnel 39 on the back surface or on a second piece. Thisscreen has similar characteristics to the double lenticular screen,i.e., high contrast and high gain, but the resolution can be increasedbecause there is no relationship between screen thickness andresolution. Screens with 0.2 mm pitch have been made.

Rear projection screens typically contain some mechanism such as minutecolloidal particles to diffuse the light into the desired viewing space.When these screens are used with high magnification systems in which theprojection beam is nearly coherent, a disturbing artifact in the form ofa speckle pattern is often observed. This speckle pattern is mostpronounced in screens with high gain.

Speckle has also been observed in microfiche and microfilm readers wherethe f/# of the beam and the magnification is high.

Speckle is most often associated with laser illumination. See forexample, D. Gabor, IBM J. Res. Develop., September 1970, pp 509-514. Itappears when random surfaces are illuminated with nearly coherent beams.

Speckle reduction has been discussed in the literature. It is well knownthat to reduce the visibility of speckle, the coherence of theillumination beam must be destroyed. This has been achieved by movingone diffusing screen with respect to another and separating thediffusing surfaces. S. Lowenthal et al., J. Opt. Soc. Am., pp. 847-851(1971) ; N. George et al., Opt. Commun., pp. 71-71 (1975); E. G. Rawsonet al., J. Opt. Soc. Am., pp. 1290-1294 (1976) and L. G. Shirley et al.,J. Opt. Soc. Am. A, pp 765-781 (1989).

We have observed that increasing the amount of diffusion and increasingthe thickness of the diffuser also can reduce the visibility of thespeckle, but on the other hand deteriorates the resolution o f thescreen.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is a principal object of the invention to reduce thespeckle pattern in rear projection screens.

It is another object of the invention to reduce the speckle pattern inrear projection screens, while maintaining the resolution, viewing spaceand gain of the screen.

It is another object of the invention to reduce the speckle pattern in arear but of the type having a front lenticular surface for spreadinglight horizontally into an audience space.

It is another object of the invention to modify the existing TIR-typerear projection screens to reduce the speckle pattern without destroyingthe resolution or significantly changing the viewing space or gain ofthe screen.

In the case of rear projection light valve systems, the light from theprojection lens subtends a very small angle at the screen. In accordancewith our invention, we have recognized that the bulk diffusing particlesdiffract this light, creating speckle. For example, for an f/3projection lens, 52 inch screen, and 1.3 inch liquid crystal lightvalve, the angular extent of the projected beam is only about ±0.32degrees. This small angle results in a large spatial coherence length atthe screen. Light diffracting from particles within the coherence lengthwill interfere coherently, resulting in speckle. We can calculate thecoherence length by using the equation

ρ=0.612λ/sin(α)  (Eq.1)

where ρ is the coherence length, λ is the wavelength of light and α isthe angular extent of the beam. For λ=0.55 microns, ρ=72 microns.Typically, the bulk diffusing particles in the screens are less thanabout 40 microns in size. Since several of these particles will fallwithin the coherence length, a speckle pattern will appear at thescreen.

By contrast, for a CRT system using 5 inch tubes, the coherence lengthat the screen is only 7 microns. Therefore, the light diffracting fromone particle is totally incoherent with respect to the light diffractingfrom the next particle. As a result, there is no interference and nospeckle.

Further in accordance with the invention, we have recognized that thespeckle in rear projection screens of the type having a front lenticularsurface can be substantially reduced by incorporating a diffractiongrating into the rear surface of the screen.

Accordingly, a rear projection screen of the invention comprises; afront surface, means for diffusing the projected light into an audiencespace, a rear phase grating surface, and a non-diffusing regionseparating the diffusion means and the phase grating.

Such diffusion means may comprise bulk diffusion, surface diffusion, orholographic diffusion, or a combination of two or more of these means.

According to a preferred embodiment of the invention, the front surfaceof the screen is defined by an array of mutually parallel lenticules forspreading projected light horizontally into the audience space, forexample, a TIR lenticular surface in which the lenticules have steepsidewalls for total internal reflection (TIR).

According to another preferred embodiment of the invention, thediffusion means comprises a bulk diffusion region between thenon-diffusing region and the lenticular surface and/or within thelenticular surface. Alternatively, such diffusion means comprises aroughening of the front lenticular surface. The phase grating is definedby a regular array of grating elements having a fixed pitch of fromabout 15 to 60 microns, where the pitch is the distance between thecenters of adjacent phase grating elements. Such an array may betwo-dimensional, e.g., an x-y matrix, but for manufacturability ispreferably one-dimensional, e.g., a linear grating of mutually parallelelements. Preferably, the elements of the phase grating have a curvedcross-section, for example, spherical or cylindrical, and have a radiusof curvature of from about 50 to 300 microns. Below a pitch of about 15microns and/or a radius of about 50 microns, the angular spread of thelight caused by the grating becomes excessive, resulting in reduced gainand resolution; above a pitch of about 60 microns and/or a radius ofabout 300 microns, resolution is reduced, and the grating becomes lesseffective as a speckle reducer.

A Fresnel lens is embodied in a separate piece which is located behindthe phase grating.

Use of such a diffraction grating to modify the existing TIR-type rearprojection screens significantly reduces the speckle pattern withoutdestroying the resolution or significantly changing the viewing space orgain of the screen.

In accordance with another embodiment of the invention, the diffractiongrating is replaced by diffusion means, either surface or bulkdiffusion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section view of a two piece double lenticularrear projection screen of the prior art, having a front piece with frontand rear lenticular surfaces, and a rear piece with a front Fresnel lenssurface;

FIG. 2 is a longitudinal section view of one piece rear projectionscreen of the prior art, having a front lenticular surface and a rearFresnel lens surface;

FIG. 3 is a longitudinal section view of a two piece rear projectionscreen of the prior art, having a front piece with a front lenticularsurface, and a rear piece having a front Fresnel lens surface;

FIG. 4 is a longitudinal section view of a two piece rear projectionscreen of the invention, having a front piece with a front lenticularsurface and a rear diffraction grating surface, and a rear piece havinga front Fresnel lens surface;

FIGS. 5 through 7 are graphical representations of line scans of theluminance outputs depicting speckle of three screens, each having adifferent screen configuration; and

FIG. 8 is a graphical representation of a line scan depicting theangular distribution of the luminance output of a grating suitable foruse in the invention, compared to a computation of expected luminanceoutput for such a grating using Fourier diffraction theory;

FIG. 9 is an enlarged portion of the diffraction grating surface of FIG.4, showing the surface contour of the gratings;

FIG. 10 is a longitudinal section view of another embodiment of a twopiece rear projection screen of the invention, in which the diffractiongrating has been replaced by a bulk diffusion region;

FIG. 11 is a longitudinal section view of another embodiment of thefront piece of a two piece rear projection screen of the invention,showing the lenticules of the front surface oriented orthogonal to theelements of the phase grating, and the diffusion regions extending intothe tips of the lenticules; and

FIG. 12 is a longitudinal section view of another embodiment of thefront piece of a two piece rear projection screen of the invention,showing a roughened lenticules front surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 4 shows the preferred embodiment of this invention, a rearprojection screen having a front piece or substrate with a frontlenticular lens array 40, a bulk diffusing region 48 and a rear surfacedefining a diffraction grating 50. The front surface is defined byindividual mutually parallel lenticular elements or lenticules 40 a,each having sidewalls 42 and tip regions 44. As shown, a linear phasegrating 50, sometimes referred to as a micro-lenticular or micro lensarray, and a bulk diffuser 48 are separated by a clear region 49. Thebulk diffuser 48 may also extend into the lenticular region 40 as far asthe tip region. The thickness of the clear region or non-diffusingregion 49 is typically about 3 mm.

FIG. 9 shows an enlarged portion of the phase grating 50, in which theindividual mutually parallel gratings 50 a have a cylindrical surface52, defined by a radius r and a pitch a. The thickness of the substrateand the grating parameters are designed to provide just enough spread ofthe light to reduce speckle while maintaining high resolution. The phasegrating typically has a pitch a of about 40 microns and a radius r ofabout 100 microns. Such a phase grating diffracts the incoming lightinto the horizontal direction with a spread typically less than ±2□.Therefore high gain is maintained.

In general, the thickness of the non-diffusing region should be betweenabout 1 and 5 mm, preferably about 3 mm.

The thickness of the bulk diffusion region should preferably be keptbelow 1 mm, e.g., 0.25-0.75 mm, for high resolution, but could beincreased to 2 mm for lower resolution applications, resulting infurther reductions in speckle.

A rear piece 52 defines a Fresnel lens, the purpose of which is to imagethe exit pupil of the projector to the viewing plane.

Another embodiment of the invention is shown in FIG. 10, in which phasegrating 50 has been replaced by a second bulk diffusion region 100. Allother features are similar to those shown in FIG. 4, and the samereference numerals have been used to indicate these features in FIG. 10.The bulk diffuser and substrate thicknesses should be designed to reducethe visibility of speckle while maintaining good resolution and highgain.

Reductions in speckle obtained using the above described embodiments ofthe invention were measured using a broad band light source toilluminate screen samples so that the angular extent of the illuminationbeam matched that of a typical light valve projector, i.e., about ±0.5.The illumination intensity was adjusted so that the DC-components of theintensity patterns were constant from sample to sample. The specklepattern was grabbed and digitized using an 8-bit black-and-white CCDcamera, and a PC equipped with an image processing board. Line scanswere used to evaluate speckle reduction. Table 1 show the screencharacteristics of each of three samples, the phase grating embodimentshown in FIG. 4, the double diffuser embodiment shown in FIG. 10, andthe prior art TIR screen shown in FIG. 3.

TABLE I SEPARATION HALF FIGURE REAR LAYER FRONT LAYER THICKNESS ANGLENUMBER Phase Grating 1.5 mm bulk 3 mm ±8° FIG. 5 diffuser 0.75 mm bulk0.75 mm bulk 3 mm ±6° FIG. 6 diffuser diffuser none 0.75 mm bulk none±8° FIG. 7 diffuser

FIGS. 5, 6 and 7 show line scans for each of these samples. As may beseen, FIG. 7 corresponding to the prior art screen of FIG. 3, shows anamplitude variation in relative intensity of luminance across the screenof up to about 40; FIG. 6 corresponding to the screen of FIG. 10, showsa significant reduction in this amplitude variation to a maximum ofabout 30 (the peak in the center of the screen represents a specularcomponent caused by weaker than normal diffusion of the rear bulkdiffuser). FIG. 5 corresponding the diffraction grating embodiment ofFIG. 4, shows further improvement, as evidenced by a reduction inamplitude variation to less than 20.

Analysis and design of the phase grating requires Fourier diffractiontheory. The surface of the grating can be described as $\begin{matrix}{{{comb}( \frac{x}{a} )} \otimes {f(x)}} & ( {{Eq}.\quad 1} )\end{matrix}$

where ${comb}( \frac{x}{a} )$

is defined as Σδ(x−a) and a is equal to the pitch of the elements. f(x)can be approximated by the 1^(st) order equation of the sag of acylindrical surface (see FIG. 8): $\begin{matrix}{{f(x)} = \frac{x^{2}}{2r}} & ( {{Eq}.\quad 2} )\end{matrix}$

where r is the radius of the element.

This surface will diffract incident light into orders at angles, θ_(m),given by the grating equation referenced by E. Hecht and A. Zajac,OPTICS, Addison-Wesley Publishing Company, P. 357, (1979),

α(sin (θ_(m)))=mλ m=1,2,3 . . .   (Eq.3)

where λ is the wavelength of light.

To calculate the intensity of each diffraction order, far fielddiffraction theory is applied. It can be shown that in the far field,the amplitude function is proportional to the Fourier transform ofA(x,y) where A(x,y) is the amplitude function of the transmitted beamand is given by $\begin{matrix}{{A( {x,y} )} = {\quad \frac{2\quad {\pi }\quad {OPD}}{\lambda}}} & ( {{Eq}.\quad 4} )\end{matrix}$

where OPD is the optical path introduced by an element $\begin{matrix}{{OPD} = {{( {n - 1} ){f(x)}} = \frac{x^{2}( {n - 1} )}{2r}}} & ( {{Eq}.\quad 5} )\end{matrix}$

Therefore, $\begin{matrix}{{A( {x,y} )} = {^{\frac{\quad {{\pi }\quad {x^{2}{({n - 1})}}}}{r\quad \lambda}} = ^{\quad \phi \quad x^{2}}}} & ( {{Eq}.\quad 6} )\end{matrix}$

where $\begin{matrix}{\phi = \frac{\pi \quad ( {n - 1} )}{r\quad \lambda}} & ( {{Eq}.\quad 7} )\end{matrix}$

The normalized intensity pattern is given by

I(u,v)={overscore (A)}(u,v){overscore (A)}*(u,v) where {overscore(A)}=FT(A(x,y))  (Eq.8)

MATLAB for Windows, a commercially available mathematical analysisprogram from The Mathwork, Inc., available was used to perform a FFT ofthe transmitted wavefront, A(x,y), and to calculate the normalizedintensity pattern, I(u,v).

For r=100 microns, a=32.5 microns, and λ=0.6328, the calculated patternis shown as the solid line in FIG. 8. Experimental data points wereobtained with a goniometer and plotted on the same figure as the smallcircles.

As may be seen, the very high correlation between the calculated andexperimental intensity pattern confirms that the grating is acting as adiffraction element.

FIG. 11 is a longitudinal section view of another embodiment of thefront piece of a two piece rear projection screen of the invention,showing the lenticules 40 of the front surface oriented orthogonal tothe elements of the phase grating 50, and the diffusion region 48extending into the tips of the lenticules.

FIG. 12 is a longitudinal section view of another embodiment of thefront piece of a two piece rear projection screen of the invention,which is similar to the embodiment of FIG. 11, except that the diffusionregion 48 has been replaced by a roughened front surface 40 b of thelenticular elements 40 a.

The invention has necessarily been described in terms of a limitednumber of embodiments. Other embodiments will be readily apparent tothose skilled in the art, and are intended to be encompassed within thescope of the appended claims. For example, the linear phase gratingelements, while shown oriented parallel to the front lenticular elementsin the embodiment of FIG. 4, could alternatively be orientedorthogonally to the lenticular elements. In addition, the rear diffusionregion shown in the embodiment of FIG. 10 could be replaced by surfacediffusion, i.e., a roughening of the rear surface of the substrate, orby holographic diffusion, which may be either surface or bulk diffusion.

What is claimed is:
 1. A Rear projection screen comprising a frontsurface, a rear surface and first diffusion means for diffusing lightinto an audience space, characterized in that the screen comprisessecond diffusion means located behind the first diffusion means and anon-diffusion region between the first and the second diffusion means,wherein the second diffusion means comprises a bulk diffusion region,said bulk diffusion region and non-diffusion region being configured toreduce speckle without substantially reducing resolution.
 2. Rearprojection screen as claimed in claim 1, characterized in that the frontsurface is defined by an array of mutually parallel lenticules forspreading light into the audience space.
 3. Rear projection screen asclaimed in claim 1, characterized in that the second diffusion meansconsists of holographic diffuser.
 4. A rear projection screen comprisinga front surface, a rear surface and diffusion means for diffusing lightinto an audience space, characterized in that the rear surface comprisesa phase grating and that the screen comprises a non-diffusing regionbetween the diffusion means and the phase grating.
 5. Rear projectionscreen as claimed in claim 4, characterized in that the front surface isdefined by an array of mutually parallel lenticules for spreading lightinto the audience space.
 6. Rear projection screen as claimed in claim 4or 5, characterized in that the diffusion means comprises a bulkdiffusion region between the front surface and the non-diffusion region.7. Rear projection screen of claim 6, characterized in that the bulkdiffusion region extends into the tip regions of the lenticules.
 8. Rearprojection screen as claimed in claim 6, characterized in that the bulkdiffusion region comprises colloidal particles suspended in the screen.9. Rear projection screen as claimed in claim 8, characterized in thatthe colloidal particles are less than about 40 microns in size.
 10. Rearprojection screen as claimed in claim 6, characterized in that the bulkdiffusion region is about 0.25 to 3 mm.
 11. Rear projection screen asclaimed in claim 5, characterized in that the diffusion means comprisesa roughening of the front lenticular surface.
 12. Rear projection screenas claimed in claim 5, characterized in that the lenticules have tipregions and steep sides for total internal reflection.
 13. Rearprojection screen as claimed in claim 5, characterized in that a lightabsorbing material is present between the lenticules of the frontlenticular surface.
 14. Rear projection screen as claimed in claim 4,characterized in that the non-diffusion region is about 1 to 5 mm. 15.Rear projection screen as claimed in claim 14, characterized in that thenon-diffusion region is about 3 mm.
 16. Rear projection screen asclaimed in claim 4, characterized in that the phase grating comprises anarray of mutually parallel linear grating elements.
 17. Rear projectionscreen as claimed in claim 16, characterized in that the phase gratinghas a pitch of from about 15 to 60 microns, where the pitch is thedistance between the centers of adjacent elements of the phase grating.18. Rear projection screen as claimed in claim 16, characterized in thatthe elements of the phase grating have a curved surface.
 19. Rearprojection screen as claimed in claim 18, characterized in that theelements have a cylindrical surface having a radius of curvature of from50 to 300 microns.
 20. Rear projection screen as claimed in one of theclaims 16 to 19, characterized in that the phase grating has a pitch ofabout 40 micros and a radius of curvature of about 100 microns.
 21. Rearprojection screen as claimed in one of the claims 16 to 19,characterized in that the elements of the phase grating are orientedorthogonal to the front lenticular elements.
 22. Rear projection screenas claimed in claim 16, characterized in that the elements of the phasegrating are oriented parallel to the lenticular elements.
 23. Rearprojection screen as claimed in claim 4, characterized in that a Fresnellens is located behind the rear surface.